1. Field of the Invention
The present invention relates to a piezoelectric element and an ink jet head, for use in an ink jet recording apparatus, and to a producing method for a piezoelectric element.
2. Description of the Related Art
The piezoelectric element is an element capable of converting an electric energy into a mechanical energy such as mechanical displacement, stress or vibration, and also of a reverse conversion, and is available for example in unimorph type and bimorph type, depending on the generated bending displacement. Such piezoelectric element is widely utilized for example in an ink jet head of an ink jet recording apparatus, a microphone, a sound generating device (such as a speaker), various vibrators, oscillators and sensors. Recently, a printer utilizing an ink jet head is widely utilized as a printing apparatus of personal computers and the like, because of a satisfactory printing performance, an easy handling and a low cost. Such ink jet head includes various types, such as one that generates a bubble by thermal energy in a liquid such as an ink and discharges a liquid droplet by a pressure wave caused by such bubble, one that discharges a liquid droplet by an electrostatic suction force, and one that utilizes a pressure wave generated by a vibrator such as a piezoelectric element.
As an ink jet head utilizing a piezoelectric element, one illustrated in FIG. 1 is known. In the ink jet head illustrated in FIG. 1, on a substrate 41 including an array of plural pressure chambers 61, a piezoelectric film 45 formed continuously in such a size as to cover two or more pressure chambers 61 is provided, across a vibration plate 42. At least either of an upper electrode 46 and a lower electrode 44, which are disposed to sandwich the piezoelectric film 45 at the upper and lower sides thereof, is formed separately for each pressure chamber 61, whereby a piezoelectric drive area is formed, in the piezoelectric film 45, for each pressure chamber 61. Such piezoelectric drive area is made smaller than the corresponding pressure chamber 61 in a planar direction parallel to the piezoelectric film 45, and is positioned with a gap to the peripheral edge and over the entire periphery of the pressure chamber 61. A predetermined voltage is applied to the piezoelectric film 45 to cause an elongation or a contraction of the piezoelectric film 45 and to induce a bending vibration in the vibration plate 42, thereby pressurizing a liquid such as an ink in the pressure chamber 61 and discharging a liquid droplet from a liquid discharge port 53.
Along with the recent pervasiveness of color ink jet printers, there are being requested an improvement in the printing performance, a higher resolution, a higher printing speed and a longer-sized ink jet head. For this purpose, it is being requested to realize an ink jet head in a miniaturized multi-nozzle head structure. In order to miniaturize the liquid discharge head, it is necessary to realize a smaller piezoelectric element for liquid discharge, and, for realizing a smaller piezoelectric element, there is required a piezoelectric element having a high piezoelectric constant, that does not lose the driving ability even when miniaturized.
As the piezoelectric film for use in the piezoelectric element, conventionally employed is a PZT type piezoelectric material which is obtained by molding a paste of PbO, ZrO2 and TiO2 powder into a sheet shape as a green sheet, followed by a sintering. However, the PZT type oxide film obtained by this method is difficult to prepare with a thickness of 10 μm or less. Also since the green sheet is sintered at a temperature of 1000° C. or higher, there results a problem that the piezoelectric film shrinks to about 70% when heated. As it is difficult to align the piezoelectric film and the structural member such as the ink chamber with a dimensional precision in the order of several micrometers, a satisfactory compact piezoelectric element has not been obtained.
Other already reported methods for preparing the piezoelectric film include processes for preparing an oxide piezoelectric film, such as a sputtering process, a chemical vapor deposition process (also represented as CVD process), a molecular beam epitaxy process (also represented as MBE process), or a sol-gel process. These methods allow to obtain a piezoelectric film of a thickness of 10 μm or less.
However, such film forming methods as sputtering, CVD process, MBE process or sol-gel process involve a drawback of being incapable of preparing a thin film comparable in characteristics to bulk ceramics. In order to solve this drawback, for example Japanese Patent Application Laid-Open No. H08-116103 proposes a method of controlling the crystal orientation to a (001) plane, and Japanese Patent Application Laid-open No. 2000-332569 proposes a producing method for an epitaxial film having a domain structure, in which (100) and (001) orientations of a tetragonal crystalline structure are mixed. Also Japanese Patent Application Laid-Open No. 2005-119166 proposes a method of forming a (100) orientation by an orientation control layer and controlling the residual stress at zero or at a compression stress.
However, even with the method of Japanese Patent Application Laid-Open No. H08-116103, the characteristics comparable to those of bulk ceramics cannot be obtained at a high electric field side so that the obtained piezoelectric element is insufficient in the performance.
Also the method disclosed in Japanese Patent Application Laid-Open No. 2000-332569 involves drawbacks of being unable to utilize a morphotropic phase boundary range (also represented as MPB range) of satisfactory characteristics and being unable to reproducibly control a proportion of (100) orientation and (001) orientation. Also in a film having an excessively large proportion of (100) orientation or namely of [100] axis, a contribution of the 90° domain effect becomes large whereby the film becomes inferior in durability in a thin film element.
The “90° domain effect” means utilizing a 90° domain in which, in a tetragonal crystalline structure, an a-domain of (100) orientation and a c-domain of (001) orientation are mixedly present. This effect is caused by reversible changes of from a-domain to c-domain and from c-domain to a-domain, and by a distortion resulting from a difference in the lattice constants, and shows a piezoelectric constant far larger than in an ordinary elongation-contraction in the polarization direction.
Also the method of Japanese Patent Application Laid-Open No. 2005-119166 is incapable of controlling (100) orientation and (001) orientation, and is incapable of forming a 90° domain having an anisotropy within the plane of the piezoelectric film.